1. Field of the Invention
The present invention relates to a semiconductor device and a method of adding a metallic element having a function for promoting crystallization to an amorphous semiconductor film and conducting heat treatment to form a crystalline semiconductor film, and manufacturing a thin film transistor (TFT) using the crystalline semiconductor film.
2. Description of the Related Art
A liquid crystal display device in which a pixel portion and a driver circuit are provided on the same substrate is used as a monitor for a personal computer (PC) and further begins to expand into ordinary households. For example, a liquid crystal display instead of a CRT (cathode-ray tube) is introduced as a television into ordinary households. In addition, a front projector for watching a movie or playing a game is introduced for recreation into ordinary households. Thus, the market size of the liquid crystal display device is rapidly being expanded. Further, the development of a system on panel in which logic circuits such as a memory circuit and a clock generating circuit are incorporated on a glass substrate is actively progressed.
When high resolution image display is conducted, the amount of information written into a pixel is increased. Further, if the information is not written for a short time, it is impossible that moving picture display is conducted for images with an enormous amount of information for high definition display. Thus, high speed operation is required for a TFT used for the driver circuit. In order to allow the high speed operation, it is required that the TFT is realized using a crystalline semiconductor film having satisfactory crystallinity by which high field effect mobility is obtained.
As a method of obtaining a satisfactory crystalline semiconductor film on a glass substrate, the present inventor et al has developed a technique for adding a metallic element having a function for promoting crystallization to an amorphous semiconductor film and then conducting heat treatment to obtain a satisfactory semiconductor film in which crystal orientations are aligned.
However, with respect to a TFT manufactured using as a semiconductor layer a crystalline silicon film obtained using a catalytic element without processing, there is a problem in that an off current is suddenly increased. The catalytic elements are irregularly segregated in the semiconductor film, and more particularly, the segregation is markedly recognized in a grain boundary. Thus, it is considered that the segregation of the catalytic elements becomes a current escape path (leakage path) and this causes a sudden increase in off current. Accordingly, it is required that, after the crystalline silicon film forming process, the catalytic elements are moved from the semiconductor film to reduce a concentration of the catalytic elements in the semiconductor film.
In order to reduce a concentration of residual catalytic elements in the semiconductor film, the following method is considered. That is, an impurity element belonging to group 15 of the periodic table (typically, phosphorus or arsenic: impurity element for imparting an n-type) and an impurity element belonging to group 13 of the periodic table (typically, boron or aluminum: impurity element for imparting a p-type), each of which has a function for moving catalytic elements, are added at a high concentration to a region which becomes a source region or a drain region of a semiconductor layer of an n-channel TFT and a p-channel TFT or a region to which catalytic elements are moved (hereinafter referred to as a gettering region), and heat treatment is conducted to move the catalytic elements. Thus, a concentration of the catalytic elements contained in a channel forming region particularly is reduced.
However, according to the method of using the source region or the drain region as the gettering region, the impurity element which imparts the n-type and belongs to group 15 of the periodic table (typically, phosphorus or arsenic) needs to be added at a higher concentration than the impurity element which imparts the p-type and belongs to group 13 of the periodic table (typically, boron or aluminum) to a region which becomes the n-channel TFT later. On the other hand, in the case of the p-channel TFT, the impurity element which imparts the p-type and belongs to group 13 of the periodic table (typically, boron or aluminum) needs to be added at a higher concentration than the impurity element which imparts the n-type and belongs to group 15 of the periodic table (typically, phosphorus or arsenic). In other words, concentrations of added impurities in the gettering regions of semiconductor layers with different conductivities are different from each other. Thus, there is a problem in that a difference is caused between the n-channel TFT and the p-channel TFT with respect to efficiency that catalytic elements uniformly contained in the semiconductor films move to the gettering regions.
Note that the present inventor et al. observe efficiency that the catalytic element moves to the gettering region using the following method.
For example, when a catalytic element (nickel) moves to a gettering region by the influence of an element added to the gettering region, it is considered that the catalytic element (Ni) is bonded to Si in the process in which the catalytic element moves from the channel forming region to the gettering region, thereby becoming NiSix (nickel silicide). As to nickel silicide, a silicon oxide film is removed by a mixture solution containing ammonium hydrogen fluoride (NH4HF2) at 7.13% and ammonium fluoride (NH4F) at 15.4% (which is produced by Stella Chemifa Corporation and whose product name is LAL500), and then a sample substrate is immersed for 40 minutes in a chemical solution mixed at a volume ratio of HF (50% in concentration):H2O2 (33% in concentration):H2O=45:72:4500 (hereinafter referred to as an FPM solution). Thus, NiSix can be selectively removed.
A portion in which NiSix has been removed becomes a pore. The pore produced by removing NiSix is observed as a black spot in a transmission mode of an optical microscope. When the number of black spots is large, it is estimated that a large number of catalytic elements (nickel) can be moved to the gettering region. In other words, it is estimated that gettering efficiency is good.
It is difficult that an impurity element for imparting a p-type is sufficiently added to the source region or the drain region of the n-channel TFT without increasing the number of steps. Thus, a difference of concentrations of the impurity elements added to the semiconductor layers of the n-channel TFT and the p-channel TFT relates to a difference of efficiencies that the catalytic element moves to the gettering region so that this becomes a cause of a problem in that a variation in element characteristics is caused.
Also, as to another problem, it is required for the p-channel TFT that a region to which an impurity element for imparting an n-type is added at a high concentration for gettering processing to a catalytic element is reversed to have a p-type (counter doping). When an n-type is reversed to a p-type in the semiconductor layer of the p-channel TFT, it is necessary to add a p-type impurity element with a concentration 1.5 times to 3 times larger than that of an n-type impurity element, and the crystallinity of the source region or the drain region of the p-channel TFT is destroyed by the counter doping. Thus, there is a problem in that inconveniences related to a TFT element such as an increase in resistivity and a reduction in on current value may be caused.